High intensity discharge lamps such as sodium, metal halide, mercury and others are commonly used sources of illumination due to their relatively high efficiencies in converting electrical input power into light output, and also due to their relatively long service lifetimes. It is well known that the efficiency of HID lamps is generally improved by operating such lamps by means of high frequency electrical input power to drive the discharge within the lamp. However, high frequency operation of such lamps brings certain associated problems, including the generation of acoustic compression waves in resonance with the natural acoustic frequencies of the HID lamp.
The use of alternating current to power an HID lamp necessarily involves a non-constant, time-varying application of electrical power to the electrodes of the HID lamp. This time-varying application of electrical power generates concomitant variations in the gas through which the electrical discharge occurs. For example, negative voltage applied to a discharge electrode will repel electrons from the vicinity thereof. Alternation of the polarity of the applied voltage during the next half-cycle will attract electrons to the same electrode. This alternative attraction and repulsion of electrons (and corresponding repulsion and attraction of positive ions) from a discharge electrode causes pressure variations in the gas in the vicinity of this electrode, substantially at double the frequency of the applied voltage, since both positive and negative applied voltages generate local regions of compression. Such pressure variations created in the vicinity of a discharge electrode will typically propagate into the gas of the HID lamp as an "acoustic wave" or an "acoustic compression wave". Thus, these acoustic waves are an inherent and unavoidable consequence of driving the electrical discharge by means of alternating positive and negative voltage being applied to the discharge electrodes. Methods for controlling these acoustic waves to avoid harmful effects on the HID lamp are the subject of the present invention.
When the discharge-induced acoustic compression waves occur at the natural acoustic frequencies of the HID lamp, acoustic resonance occurs. The phenomena of acoustic resonance essentially generates standing pressure waves within the HID tube. Such waves can cause the light from the lamp to flicker; cause the arc within the tube to warp, bend or become extinguished; or in extreme cases cause the arc to contact the walls of the HID lamp and damage or destroy the tube itself. Even modest variations in spacial or temporal light intensity are unacceptable in many applications of HID lamps in which focusing of the light is necessary. Other deleterious effects of acoustic resonance may considerably shorten the service lifetime of the lamp.
The precise frequencies at which acoustic resonance occurs is a complex function of the composition, temperature and pressure of the gas within the HID tube, and the geometry of the tube itself. In addition, the composition, temperature and pressure of the gas varies from place to place within the tube, being typically hotter and less dense near the center of the arc while cooler and more dense near the walls of the tube. Adding further to the complexity of acoustic resonance is the fact that the properties of the tube and the gas are not constant over time. Tube electrodes will typically change their geometry over the lifetime of the lamp as they are subjected to numerous hours of electrical discharge and bombardment by ions, electrons and neutral species from the gas of the HID tube. The composition of the gas will similarly change over time as chemical processes within the HID gas proceed over many hours of operation. Practical manufacturing tolerances also lead to variations in tube geometry from lamp to lamp, even when new. All these factors accumulate so as to make it exceedingly difficult to predict with any reasonable precision the acoustic resonance frequencies of a particular HID tube, or to predict how such acoustic resonance frequencies will change over the service lifetime of the lamp. In general, acoustic resonance frequencies tend to occur in the range above about 10 KHz for typical HID lamps, increasing thereby the complexity in obtaining efficient, high frequency operation of such lamps.
Despite the difficulties in predicting acoustic resonance effects, several attempts have been made to avoid acoustic resonance and the accompanying deleterious effects on the operation of the HID lamp.
The work of Wada et. al. (U.S. Pat. No. 4,724,361) involves a careful exploration of the frequency regions at which acoustic resonance occurs for various tube geometries. These inventors suggest the use of certain HID tube geometries, with special attention to the design in the region of the tube end caps, so as to minimize the frequency range in which resonance occurs. Such designs presumably make it easier to avoid the remaining acoustic resonant frequencies in the operation of the HID lamp.
Davenport (U.S. Pat. No. 4,170,746) has carefully evaluated the frequency ranges at which acoustic resonance occurs for the special case of miniature high pressure metal vapor lamps (typically less than 1 cubic centimeter in discharge volume). Davenport finds resonance-free regions between approximately 20 KHz and 50 KHz for such lamps and suggests operation at these frequencies as a solution for acoustic resonance, at least for the miniature lamps included in his studies.
An approach to avoiding acoustic resonance by choosing a suitable geometry for the HID tube has certain serious drawbacks. In typical operation, square wave pulses have often been used to drive the HID discharge. Such pulses contain numerous harmonic components, increasing markedly the chance that one or more of such frequencies will occur at an acoustic resonance with the tube. Also, as noted above, the tube geometry and acoustic propagation properties are not constant over the service lifetime of the tube. Successful avoidance of acoustic resonance at one time may not necessarily lead to avoidance of acoustic resonance at a later time during the service life of the tube.
For these reasons, other workers in the field have looked to avoid acoustic resonance by means other than careful selection of the geometry of the tube, and/or careful selection of driving frequencies. For example, in the work of Bonazoli et. el. (U.S. Pat. No. 4,373,146), a square-wave driving pulse is frequency modulated to sweep the applied frequency from about 20 to 30 KHz. The idea here is to avoid the detrimental effects of acoustic resonances by sweeping the driving power quickly through any acoustic resonance frequency which may occur in the spectrum of the driving power of the lamp. The result is presumably that acoustic resonance waves do not build up to large amplitudes since power is delivered to the tube at any one resonant frequency for only brief periods of time. However, the use of square waves (although modulated) necessarily provides a reasonably broad spectrum of frequencies at which input power is delivered to the tube, thus potentially exciting many acoustic resonances within the HID tube. Sweeping or modulating a square wave, or sawtooth, or other waveform, will not readily avoid the generation of a rich spectrum of acoustic frequencies within the HID discharge gas.
Kachmarik et. al. (U.S. Pat. No. 5,357,173) use a square wave pulse with carefully selected pulse widths. Their intent seems to be to tailor the pulse harmonics such that low amplitudes (readily damped within the HID tube) occur at the acoustic resonant frequencies of the particular HID tube.
All of the above approaches to dealing with acoustic resonance in HID tubes depend upon some previous knowledge of the acoustic resonant frequencies to be encountered, allowing either the tube geometry, the driving power waveform, or perhaps both, to be adjusted to reduce or avoid those problems brought by uncontrolled acoustic resonance. We mention above that such approaches are problematic in so far as the acoustic resonance frequencies of any particular HID tube are not generally expected to remain constant over the service lifetime of the tube, or from tube to tube. Even successful avoidance of acoustic resonance in new tubes may prove ineffective after some hours of use. Tube to tube variations are also inherent in any practical manufacturing process, leading to different acoustic resonant frequencies for different samples of the same lamp. Thus, acoustic resonance phenomena appear as yet another factor tending to degrade the performance and reduce the useful service lifetime of such HID tubes.
Another approach to dealing with acoustic resonance has been to look for beneficial effects of such resonance, and design HID tubes intentionally to generate acoustic resonance to utilize such beneficial effects. The work of Roberts (U.S. Pat. No. 4,983,889) uses standing waves generated by acoustic resonance as a means to achieve mixing of components in a multi-component HID tube. The work of Dakin et. al. (U.S. Pat. No. 5,306,987) uses intentionally generated acoustic resonance waves within an HID tube (in conjunction with a suitably modulated driving waveform), to achieve stability of the arc. However, as noted above, it is not simple to maintain stability in acoustic resonance (either avoiding it or generating it intentionally) due to the varying resonant frequencies occurring over the service lifetime of the HID lamp as well as lamp to lamp variations.
In contrast to much of the prior art, the present invention is not based upon avoidance of the acoustic resonant frequencies of the particular HID lamp. Rather, the present invention makes use of the natural damping mechanisms of the HID tube. Acoustic compression waves will be subject to two general classes of damping within the HID tube. One mechanism of damping is "viscous damping" in which the intermolecular, interatomic and interionic forces between the electrons, atoms, ions and molecules within the tube lead to a finite viscosity in the discharge gas. Propagation of an acoustic compression wave through such a viscous medium will be subject to damping due to the energy extracted from the wave in moving one species against another.
The second general class of damping results from the impact of the acoustic compression wave with the wall of the tube, as well as with other structures (electrodes, end caps, etc.) within the tube. For economy of language, we will refer to all such solid surfaces onto which an acoustic wave might impinge as "walls". Transfer of energy from the acoustic wave to the tube wall results in loss of energy from the wave and, hence, damping. Typically, such "wall effect" damping will dominate viscous damping under conditions of pressure, temperature, composition and geometrical configuration typically found in HID tubes.